Anomalous kinetic hydrogen isotope effect in the oxidation of formate by bromine: Tunneling via an intermediate
✍ Scribed by M. A. Brusa; A. J. Colussi
- Publisher
- John Wiley and Sons
- Year
- 1982
- Tongue
- English
- Weight
- 400 KB
- Volume
- 14
- Category
- Article
- ISSN
- 0538-8066
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✦ Synopsis
Rates of oxidation of XCOO-(X = H, D) by Brz in acid aqueous media were measured between 274 and 332 K. The derived Arrhenius parameters for both reactions log k ~( M -l S-') = (11.81 f 0.10) -(14.33 f 0.13)/0 log ~D ( M -' S-') = (13.77 f 0.13) -(17.62 f 0.04)/0 where 0 = 4.575T X kcal/mol, with ( k ~/ k & 9 8 ~
= 2.85, reveal a primary isotope effect, but the difference ( E D -E H ) = 3.29 kcal/mol and the ratio A D / A H = 91 fall beyond the limits imposed by semiclassical transition-state theory, suggesting tunneling or a multiple-stage mechanism. However, it can be shown that either tunneling in a single step or a three-step, internal return mechanism can be ruled out as alternative models, since both require unreasonable kinetic parameters to fit the data. The simplest scheme accounting for the present observations involves tunneling in the decomposition of a charge transfer complex in equilibrium with the reactants.